The present invention relates to a zoom lens suitable for a compact camera, in which there is less restriction to a backfocus, and more particularly to a zoom lens that is compact but has a high zoom ratio.
The following five types of zoom lens systems have conventionally been used in compact cameras:
(I) A two-group system of the telephoto type composed of a first lens group having a positive focal length and a second lens group having a negative focal length and including a diaphragm stop. Such lens systems are disclosed in Unexamined Published Japanese patent application Nos. 56-128911, 57-201213, 60-48009, 60-170816, 60-191216, 62-90611, 62-264019, 62-113120 and 63-155113. Also, the present applicant proposes such a two-group system in Japanese application Nos. 62-214709, 63-19092 and U.S. patent application Ser. No. 328,265 filed on Mar. 24, 1989.
(II) A three-group system that is a modification of the two-group system (I) and which is composed of a first lens group (master lens) having a positive focal length, a second lens group having a positive focal length, and a third lens group having a negative focal length and including a diaphragm stop. Such a lens system is disclosed in Unexamined Published Japanese patent application No. 58-184916.
(III) A four-group system composed of a first lens group having a positive foal length, a second lens group having a negative focal length, a stop diaphragm after the second lens group, a third lens group having a positive focal length and a fourth lens group having a negative focal length. Such a lens system is disclosed in Japanese Unexamined patent application No. 60-57814.
(IV) A three-group system which is a modification of the four-group system (III) in which the system is composed of a first lens group having a positive focal length, a second lens group having a positive focal length and a third lens group having a positive focal length. The second lens group includes a stop diaphragm and is composed, in order from the object side, a negative sublens group 2a and a positive sub-lens group 2b. During zooming, the sub-lens groups 2a and 2b are moved in unison with each other. See Japanese Unexamined Patent Publication No. 62-78522 hereinafter referred to as system (IV).
(V) A high zoom ratio four-group or three-group system which has the same arrangement of lens components and a stop diaphragm as that of the systems (III) or (IV) but has a zoom ratio of about 2.1 to 2.8 times (the zoom ratio of the systems (III) and (IV) is about 1.6 times). Such a system is disclosed in, for example, Japanese Unexamined Patent Publications Nos. 63-43115, 63-153511, 63-157120, 63-159818 and 63-161423 which will be hereinafter referred to as system (V).
The above-described conventional zoom lens systems, however, have certain problems as discussed below.
In systems (I) and (II), the zoom ratio is smaller than 2. If the larger zoom ratio is obtained by the concept of the systems (I) and (II), the movement of each lens group would be abruptly increased. It is thus impossible to make the systems small in size due to their structural limitation.
The systems (III) and (IV) are small in amount of movement. However, the zoom ratio thereof is smaller than 2. If the zoom ratio is increased under the concept of the systems, a diameter of the front lens element and the overall length of the systems are abruptly increased, resulting in enlargement of the lens systems. This is undesirable for a compact camera.
The three- or four-group system (V) is characterized in that the zoom ratio exceeds 2. However, the four-group system is complicated in structure so that respective four groups are moved independently of each other. In addition, in this system, a stop diaphragm is interposed between the second and third lens groups that are largely affected by manufacture errors. It is therefore difficult to ensure manufacture precision of the second and third lens groups that are very sensitive. On the other hand, in the three-group system, since the stop diaphragm is disposed in the second lens group, it is difficult to enhance the precision of the sub-lens groups 2a and 2b. Thus, this system also suffers from a disadvantage that it is difficult to manufacture the system. Also, either of the three-and four-group systems is not compact in size.